Resonant converters are becoming more and more popular in the field of power conversion, because they can be operated at high frequencies and offer low switching losses.
A resonant converter includes at least one half-bridge circuit with two switches that generates a square wave voltage from a DC input voltage. The square wave voltage is supplied to a resonant circuit, and the voltage across one of the circuit elements of the resonant circuit is rectified in order to provide a DC output voltage. The square wave voltage causes an oscillating voltage in the resonant circuit. This oscillating voltage allows the switches of the half-bridge circuit to be switched with zero voltage, which helps to keep switching losses low, even at high switching frequencies.
The resonator of the resonant converter may include a transformer with a primary winding and a secondary winding. In this type of converter, usually a voltage across the secondary winding is rectified employing a rectifier network connected between the secondary winding and output terminals. At the output terminals the output voltage is available. A series resonant converter has a series LC circuit connected in series with the primary winding of the transformer, and an LLC converter additionally connected to the LC circuit includes a further inductance connected in parallel with the primary winding of the transformer.
The input power of a resonant converter and, therefore, the output voltage can be regulated by suitably adjusting the switching frequency of the half-bridge circuit. The resonant converter can be operated at frequencies above a resonant frequency of the resonant network and can be operated at frequencies below the resonant frequency, wherein the operation mode influences the current waveform in the rectifier network.
In a series resonant converter and in an LLC converter a voltage ringing with high voltage peaks may occur on the secondary side rectifier network, especially in those cases in which the rectifier network has a center-tap topology. This ringing may result from parasitic oscillator circuits that may include parasitic capacitances and parasitic inductances of the oscillator. In conventional converters the circuit elements in the rectifier network are selected such that their voltage blocking capability is high enough so as to withstand these high voltage peaks. However, this increases the costs and reduces the overall converter efficiency, because circuit elements with a higher voltage blocking capability usually have higher conduction losses.
There is, therefore, a need to provide a resonant converter, in particular, a series resonant converter or an LLC converter, in which voltage ringing on the secondary side of the transformer is largely prevented or at least partially suppressed.